It is generally known that, when bending in V shape a sheet-like workpiece by using a bending machine such as press brake, etc., the behaviour relating the plastic deformation of the workpiece changes according to the characteristic values of the material and, for that reason, the bending angle greatly varies from lot to lot even with one same material depending on the variations of characteristic values of that material. From this fact, it is extremely difficult to control the driving amount of the driving die and, in actual bending, it was normally the case that this control is made by a sixth sense of a skilled operator.
To cope with such problems, a variety of press brakes are proposed which are designed to detect the bending angle of the workpiece during the bending, and control the final driving amount of the driving die based on that detected bending angle, and put to practical use.
For example, the type proposed in Japanese Patent Laid-Open Publication No. 6-328136 (1994) is realized in such a way as to once unload by moving and separating the upper and lower dies half way during the bending process, determine the spring back angle of the workpiece by measuring the bending angle of the workpiece before and after this unloading, and then calculate the final driving amount from the spring back angle determined this way and the bending angle of the workpiece before moving and separation of the die. Moreover, the type proposed in Japanese Patent Laid-Open Publication No. 7-265957 (1995), for example, is realized in a way to measure the bending angle of the workpiece in the state in which the die pressurizes the workpiece during the bending, and calculate the final driving amount of the driving die based on the result of this measurement. In that case, it is so arranged that the data of spring back angle is stored in advance in layers prepared for respective kinds of material and thicknesses of workpiece, and the relation of driving amount to the bending angle of the workpiece is corrected by using this stored data.
However, a problem with the method disclosed in said former publication (Japanese Patent Laid-Open Publication No. 6-328136 (1994)) is that, because the upper and lower dies are once relatively moved for separation on the way of the bending process, the workpiece may possibly fall down at the time of unloading causing displacement of the contact point between the die and the workpiece, in the case of a workpiece of asymmetric shape, and it becomes necessary to prevent falling of the workpiece by some means or another.
On the other hand, the method disclosed in said latter publication (Japanese Patent Laid-Open Publication No. 7-265957 (1995)) can solve said problem of falling of workpiece, because the bending angle of the workpiece is measured in pressurized state. However, since the spring back angle .theta.s has some slight variations due to variations material characteristic values, as shown in FIG. 12, it becomes necessary to either estimate or detect this spring back angle with good accuracy, to achieve bending of higher accuracy. This FIG. 12, showing the spring back angle at various bending angles in cold rolled steel sheet of different lots of same kind for 6 different kinds, indicates that the spring back angle deviates by approximately .+-.0.3.degree. because of variations from lot to lot in the material characteristic.
Moreover, with the method described on this latter publication, there are cases where the accuracy of estimation of final driving amount becomes poor, because it simply corrects the relation of the driving amount to the bending angle of the workpiece stored in advance. Namely, as shown in FIG. 13, the relation of the driving amount of the driving die to the bending angle is a substitute characteristic of mechanical properties of the workpiece material submitted to bending, and the bending accuracy is largely influenced by a change of this workpiece material. Therefore, although the bending can be made with good accuracy even with this method in the case of bending of a material having characteristics close to the mechanical properties of the material stored in advance, there is a risk of poor bending angle accuracy due to a large difference between the driving amount and the bending angle in the case of materials with much different mechanical properties.